Photoelectric sensors are widely used in industry for a variety of measurement and control functions, based upon breaking or restoring of a light beam emitted and received by the sensor system. Examples include counting objects moving along a conveyor, determining the size of objects, detection of a machine part moving into a predetermined position, and countless other applications in many different industries.
Photoelectric sensors can be of two general types. In one type, a single housing contains both a light emitter and a light detector. A reflective target is used to rebound the beam from the emitter back to the detector across the path of objects of be detected. The other type employs an emitter and a detector in separate housings which are placed on opposite sides of the objects' path, with the beam being transmitted directly from the emitter across the path to the detector.
In all types of photoelectric systems some method of indicating accurate alignment of the optical path is required to aid in the correct positioning of the elements for proper operation. A common alignment technique utilizes an indicating lamp or LED with circuitry that illuminates the indicator when the scanning beam from the emitter is aligned with the receiver. The lamp or LED turns off when the beam is broken or misaligned with the receiver. When installing a system, the emitter, receiver and reflecting target, if any, are positioned so that the indicating lamp or LED turns on, at which time the components are secured in place.
While this type of mechanism is useful in achieving alignment in many instances, it is unfortunately subject to certain disadvantages and difficulties. The most common difficulty is that the indicator does not signal the degree of beam alignment with the receiver, but instead only indicates that the alignment is adequate to permit light beam detection under the conditions prevailing at that time. However, it is common to have a situation where the alignment is only marginal at the time of the installation, and thereafter a small amount of vibration, dirt buildup or drift of the amplifier electronics causes a system malfunction.
U.S. Pat. No. 4,356,393 discloses an alignment system that provides an indication of the degree of alignment of the photo-sensing components. Specifically, a light or sound source is pulsed at a frequency that varies with the intensity of the received light from the sensing beam. Thus the greater the frequency, i.e. the faster the light flashes or the beeps occur, the better is the alignment of the sensing components.
Another alignment approach is found in U.S. Pat. No. 5,243,181 in which the intensity of the received sensing signal reducing towards the detection threshold of the system is detected by a logic circuit which provides an indication of the marginal signal. This process only operates near the threshold of detection and is facilitated by the inevitable noise in any low level signal.